The manufacture of such envelopes is an important step in the manufacture of various kinds of electrical and electronic devices incorporating elements, energizable by electrical or electromagnetic signals. Such devices include for example display devices of gas-discharge and other types, electrical components comprising encapsulated microelectronic circuitry, semi-conductor devices, photovoltaic cells or photoresistive cells, and energy conversion devices such as devices with encapsulated solar cells.
Depending on the nature of the device, the gaseous substrance to be sealed in the envelope may be a gas different in composition and/or pressure from the ambient atmosphere.
The employment of vitreous material in the manufacture of such envelopes is often necessary or desirable by reason of its chemical and physical properties. Transparent glass is a particularly advantageous material where transparency to light or other electromagnetic radiation is required, as for example in display devices and solar panels and other devices incorporating elements which must be effectively exposed to light or infrared radiation.
The hermetic sealing of one glass envelope component to another envelope component of glass or other material while the components are enclosed in an hermetically sealed chamber and the correct within-envelope environment is maintained, involves various problems. The chamber precludes access to the assembly for sealing it by hand. Heating of the assembled components to very high temperatures should be avoided. It would require large amounts of energy under those conditions, and in some cases devices enclosed in the envelope would be liable to be impaired.
French Pat. No. 2,166,229 describes a method of manufacturing the sealed glass envelope of a gas-discharge display panel wherein envelope components in the form of glass sheets are bonded together by means of a vitreous sealant. This sealant is applied in the form of a paste to the facing margins of the two sheets of glass, the sheets are assembled with an intervening spacer and placed in a vacuum chamber, the pressure in the chamber, and consequently in the envelope, is reduced to the required sub-atmospheric value, and the entire chamber is then heated in a furnace to about 500.degree. C. to fuse the glass sealant so that it bonds the sheets together on cooling. This high temperature heating takes considerable time and would make the method unsuitable if devices susceptible to damage by heating to such higher temperatures had to be enclosed in the envelope.
U.S. Pat. No. 3,926,502 proposes to form a peripheral joint between vitreous envelope-forming components by means of a low-melting glass while leaving an opening via which evacuation of air and injection of a required fluid can take place, and to seal that opening after such injection by means of a low-melting metal. The evacuation and injection take place in a chamber sealed from atmosphere. There is no teaching how to locate and fuse the low-melting metal while the envelope is in the chamber.
U.S. Pat. No. 4,029,371 proposes employment of a glass sealant for forming a peripheral joint between envelope-forming components while leaving an opening via which air can be withdrawn from the envelope and replaced by another gas, and the subsequent sealing of said opening by a fusible glass plug having a softening point below that of the sealant used for the envelope joint. In order to permit the sealing up of the evacuation and gas-refill opening, the unsealed envelope is placed in an evacuation and refill chamber while orientated with the said opening uppermost and with the fusible glass plug resting in position over that opening. After establishing the correct gaseous environment in the envelope the plug is heated and fused in situ. Due to the assembly conditions required to supporting the plug preparatory to fusion and the difficulty of forming a reliable seal by fusion of a deposited solid glass plug, this method is not able to meet the requirements in view in making the present invention. There is moreover no teaching in the United States patent how the localised heating should be performed.